Our objectives are to qualitatively identify and quantitatively define changing cell surface properties such as adhesiveness, motility, and cell contact behavior of pre-implantation mouse embryos as a function of their developmental age. The developmental period of most of our experimetal focus is the morula - to - blastocyst transformation (also termed blastulation or cavitation) which occurs between 60 to 90 hours post coitum. Presently our research includes: (1) development of an assay to measure surface adhesiveness of intact but denuded (removal of zona pellucida) mouse embryos versus their developmental age, (2) determination of dose-response effects of cytochalasin B on developing mouse embryos in vitro, (3) acquisition of quantitative ultrastructural data on the t-W32/t-W32 (also known as t-12/t-12) lethal mutant which undergoes developmental arrest at the morula stage especially as these data relate to cell surface/cell cortex phenomena, and (4) further characterization of the generation of the t-W32/t-W32 lethal syndrome especially in terms of the precise chronology of abnormal events and the clarification of these events as primary, secondary, or tertiary genetic defects. BIBLIOGRAPHIC REFERENCES: Granholm, N. H. and Brenner, G.M. 1976a. Effects of cytochalasin B on the morula-to-blastocyst transformation and trophoblast outgrowth in the early mouse embryo. Exptl. Cell Res. In Press. Granholm, N. H. and Draayer, H. A. 1976. Effects on cell junctions and cellular ultrastructure following long term 4.0 ug/ml cytochalasin B treatments of eight-cell ICR mouse embryos. S.D. Acad. Sci. Proc. In Press.